This invention relates to a steel reheating furnace, such as a slab reheating pusher furnace and/or a walking beam furnace, and in particular to a furnace including a soak zone in which the slabs are actively heated from below to eliminate skid marks.
Reheating furnaces are used to raise the temperature of steel ingots, billets, slabs and the like in the course of processing until they are sufficiently hot to be economical for reduction by rolling, forging or drawing into a desired section. In one continuous-type of reheating furnace, the workpieces are pushed through the furnace on skid rails and heat is applied above and below the workpiece such that the workpieces are heated to rolling temperature. The skid rails locally restrict heat that would otherwise fall on the workpiece, thereby causing a temperature differential in the material which is sometimes visually identifiable on discharge and termed a skid mark. Importantly, rolled steel uniformity is adversely affected by temperature differentials such that extended processing of the workpieces may be necessary to eliminate differences in material.
If the material in the vicinity of the skid mark is not hot enough at discharge, the forces needed to complete deformation of the material during a rolling operating may exceed the rolling forces that the roll mechanism can supply and system damage may result. Even if the steel is sufficiently hot to allow successful rolling, the load seen by the rolls may vary significantly during rolling, requiring a control system when metal thickness or "gauge" control is important. However, because much of the steel rolled to strip is supplied under minimum gauge requirements, rolling mill controls are frequently set up to maintain this minimum gauge condition, resulting in overgauge conditions in the vicinity of the skid mark. This is a material loss to the operation in that excess material will be shipped.
U.S. Pat. No. 3,642,261 discloses a skid rail having support portions laterally offset relative to a vertical plane through the rail. Early efforts to improve steel quality resulted in a change from uninsulated water cooled rails to insulated rails. The lower temperature of the uninsulated skid rail was a heat sink and resulted in a heat loss. The insulated rail increased the temperature of the rail surface but also increased the outer dimensions of the support system and thus increased radiative shading. It is believed that rail surface temperature is not as significant in skid mark generation as the shielding of the slab from furnace radiation. The mechanism of heat transfer in the vicinity of the rail includes direct radiation from the furnace wall and flame or products of combustion, radiation interchange with the support, diffusion within the slab and some small amount of conduction. The reheating process is sensitive to additional furnace residence time needed for reducing or minimizing temperature gradients due to reduced heat transfer in the vicinity of the skids.
It is an object of this invention to provide a reheating furnace which reduces or eliminates the cooler regions in the slabs known as skid marks.
Another object of this invention is establishment of a concentrated energy flow in the vicinity of the skid support system by provision of active preferential heating, prior to discharge.
A further object of this invention is provision of a furnace heating system which reduces the required residence time in a reheat furnace by reducing thermal gradients formed during processing through the use of active heating elements.
Yet a further object of this invention is provision of pressurized heating means which significantly improves the convective mode of heat transfer.
Another object of this invention is provision of a pressurized heating means which may be used to enhance the radiative heat transfer mode.
In accordance with this invention, there is provided a furnace having a chamber for heating workpieces which are to be advanced through the furnace from an inlet and delivered to an outlet. In particular, the chamber comprises a soak zone adjacent to the outlet, means for supporting the workpieces including a plurality of laterally spaced, longitudinally extending, support rails, each rail having a first and second portion for supporting first and second areas on the workpieces with each first portion being laterally offset to its second portion and proximate to the outlet, and heating means in the soak zone for actively heating the workpieces, including a plurality of longitudinally extending first heating elements for directing heat at the second areas only when the first areas are supported by the first portions. Generally, the rails and the heating elements are disposed, respectively, in parallel planes, the elements being superposed by the respective skid marks previously formed by the rail second portions engaging the second areas on the workpieces, immediately prior to the workpieces being pushed on to the first portions. The heating elements are longitudinally apertured and direct heating gases at the first areas, each element either being supplied by a compressed gas/air mixture external to the furnace and combusted therewithin, or supplied under pressure and combusted within the furnace as a result of the elevated temperatures provided in the soak zone.
In another embodiment according to this invention, each rail includes a third portion which is proximate to the inlet and axially aligned with the first portion proximate the outlet, the second portion being intermediate and laterally offset to the first and third portions, the respective aligned first and third portions supportingly engaging the first areas on the slab bottom and second portions supportingly engaging the second areas on the slab bottom. Advantageously, this can be extended to a fourth, fifth portion, etc.
Heating means for actively preferentially heating the slabs are disposed in a horizontal plane vertically below the rails and include the above-stated first heater elements, associated with the first portions, and a like array of second heater elements that are associated with the second portions. The second heater elements are superposed by the respective skid marks previously formed by the rail third portions engaging the first areas, immediately prior to the workpieces being pushed on to the second portions. The first heater elements generally prevent skid marks from building up in the second areas when the second areas were supported by the second portion. The second heater elements eliminate skid marks from the first areas from when the first areas were supported by the third portions.
Advantageously a furnace arrangement that preferentially heats the workpieces reduces or eliminates thermal differentials which could damage rollers and/or contribute to overgauge conditions.
Localized heating can provide for a reduction in the length of the soak hearth and pressurized combustion provides enhanced heat transfer, such as high rates of convection. A higher thermal head (through higher flame temperature) may be utilized to achieve increased radiative heat transfer.
Suitably offsetting active heating members relative to the skid rails will obviate accumulation of scale and skid marks.
Reduced residence time in the furnace has the potential of increasing fuel economy and decreasing scale loss.
Additional objects and advantages of the present invention will become apparent from reading the detailed description of the preferred embodiments which makes reference to the following set of drawings.